The invention relates to wireless communications and, more particularly, to wireless communications using spread spectrum techniques.
Wireless communication systems are widely deployed to provide various types of communication, such as voice and data communications. These systems may be based on a variety of modulation techniques, such as code division multiple access (CDMA) or time division multiple access (TDMA). A CDMA system provides certain advantages over other types of systems, including increased system capacity.
A CDMA system may be designed to support one or more CDMA standards such as (1) the xe2x80x9cTIA/EIA-95-B Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular Systemxe2x80x9d (the IS-95 standard), (2) the standard offered by a consortium named xe2x80x9c3rd Generation Partnership Projectxe2x80x9d (3GPP) and embodied in a set of documents including Document Nos. 3G TS 25.211, 3G TS 25.212, 3G TS 25.213, and 3G TS 25.214 (the W-CDMA standard), (3) the standard offered by a consortium named xe2x80x9c3rd Generation Partnership Project 2xe2x80x9d (3GPP2) and embodied in a set of documents including xe2x80x9cC.S0002-A Physical Layer Standard for CDMA2000 Spread Spectrum Systems,xe2x80x9d the xe2x80x9cC.S0005-A Upper Layer (Layer 3) Signaling Standard for CDMA2000 Spread Spectrum Systems,xe2x80x9d and the xe2x80x9cC.S0024 CDMA2000 High Rate Packet Data Air Interface Specificationxe2x80x9d (the CDMA2000 standard), and (4) some other standards.
CDMA systems use a slotted paging channel to notify wireless communication devices (WCDs) of incoming calls and to send other control information. To conserve power, idle WCDs, e.g., WCDs that are not currently in a call, xe2x80x9cwake upxe2x80x9d during periodic allocated time slots to monitor the paging channel for messages that may indicate an incoming call or convey other information. During the rest of the time, idle WCDs shut down most of their circuits and enter a xe2x80x9csleepxe2x80x9d state. The intervals at which a WCD monitors the paging channel may be set via service programming. For example, the WCD may be programmed to monitor the paging channel once every 1.28 seconds. Longer intervals conserve more power, but increase the time involved in paging a WCD for an incoming call. The slotted paging mode allows the WCD to remain in a power-efficient sleep state most of the time, increasing standby time.
In order to determine whether the slotted paging channel is carrying a message for a particular WCD, the WCD must demodulate the entire message. Messages transmitted using the paging channel are at least 20 milliseconds (ms) in duration, and more often approach 40-50 ms. As a result, when the WCD wakes up to receive a message using the paging channel, the WCD must wake up for at least 20 ms, for example, every 1.28 seconds. The WCD incurs additional overhead both before and after receiving the message. Before receiving the message, for example, the WCD must warm up the RF circuit, and must resynchronize its timing and frequency references with a base station. In addition, after receiving and demodulating the message, the WCD must process the message to determine whether the message is destined for the WCD.
For improved power conservation, the CDMA2000 and W-CDMA standards specify an alternate paging channel that transmits paging indicators. These indicators are typically less than 1 ms in length and indicate that a WCD has a paging message in the paging channel. The time involved in demodulating paging indicators is decreased compared to the time involved in demodulating a paging message, and the WCD can remain in the sleep state for a greater portion of each cycle when the paging indicator indicates that the WCD does not have a paging message in the paging channel. Standby time is thereby improved without significant degradation of paging channel performance. In the CDMA2000 standard, this alternate paging channel is known as the quick paging channel (QPCH). In the W-CDMA standard, the alternate paging channel is known as the paging indicator channel (PICH).
In order to guarantee satisfactory performance in the quick paging channel, conventional WCDs set a QPCH demodulation threshold such that the probability of detecting a transmitted paging message is high. With the threshold thus set, the probability of missing a paging message due to errors in demodulating the quick paging channel is low. This bias in the threshold setting, however, may cause a significant increase in falsely detecting a transmitted paging message when no such message was transmitted.
False alarms can cause the WCD to spend significant amounts of time demodulating paging messages that do not actually exist. With the time involved in demodulating the quick paging channel, e.g., the time involved in warming up the RF circuit, resynchronizing the timing and frequency references of the WCD with the base station, QPCH demodulation, and post-processing, high false alarm rates may make the overall average wakeup time with the alternate paging channel enabled worse than that realized with the slotted paging channel. Accordingly, the power conservation benefits that may otherwise be realized by using a quick paging channel may be reduced or even eliminated by a high false alarm rate.
In general, the invention improves the standby time of a wireless communication device (WCD), such as a mobile phone, by using an alternate paging channel to receive paging indicators when the alternate paging channel is of sufficient condition. In particular, a channel estimator predicts the condition of the alternate paging channel, i.e., the strength of the QPCH signal, based on historical measurements of the pilot signal strength, which is an indicator of the condition of the alternate paging channel. When the QPCH signal is weak, the alternate paging channel is disabled to avoid false alarms, and the slotted paging channel is used to receive paging messages without monitoring the alternate paging channel for a paging indicator. On the other hand, when the QPCH signal is strong, the probability of false alarms is lower, and the alternate paging channel is enabled.
The invention may provide a number of benefits. By using the alternate paging channel to receive paging indicators only when the signal strength is sufficiently high, a WCD may take advantage of the short duration of paging indicators carried on the alternate paging channel while reducing susceptibility of false alarms. For example, the WCD may receive a paging indicator that indicates that no paging message is present and return to a sleep state rather than remain awake to demodulate a paging message when none is present. Thus, the wireless communication device may remain in a sleep state for greater periods of time during slot cycles in which no paging message is present. As a result, standby time may be significantly improved.
According to various embodiments of the invention, a paging channel demodulator can operate in either of two modes. When an estimated signal strength of a quick paging channel is at least a threshold value, the paging channel demodulator operates in a first mode in which it demodulates a paging indicator received via a quick paging channel. When the estimated QPCH signal strength is less than the threshold value, the paging channel demodulator operates in a second mode in which it demodulates a paging message received via a slotted paging channel.
In one embodiment, the invention is directed to a demodulation method implemented in a spread spectrum system. According to the method, a signal strength in a paging channel is predicted, and the paging channel is selectively enabled based on the predicted signal strength of the paging channel. In particular, the paging channel may comprise an alternate paging channel, such as the Quick Paging Channel (QPCH) of CDMA 2000 or the Paging Indicator Channel (PICH) of W-CDMA, and may be enabled when the predicted signal strength is at least a threshold. This alternate paging channel carries paging indicators that are shorter than the paging messages used to indicate an incoming call. The alternate paging channel may be disabled when the predicted signal strength is less than the threshold, and a slotted paging channel may be monitored for a paging message. Predicting the signal strength may comprise estimating the signal strength as a function of historical pilot Ec/Io information representative of a pilot signal strength and a total received power spectral density in a carrier band.
Additional details of various embodiments are set forth in the accompanying drawings and the description below. Other embodiments, for example, include processor-readable media and apparatuses embodying these methods. Other features and advantages will become apparent from the description and drawings, and from the claims.